<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
        var year = prompt('请输入出生年份');
        var age = 2021 - year; //year取过来为字符串型 但此处用减法 有隐式转换
        alert('你今年已经' + age + '岁了');
    </script>
</head>

<body>

</body>

</html>